Say you have a device that projects the screen on a skyscraper in Manhattan, visible by tens of thousands of people in real time. You can do all the cryptography you want, I would argue that you fundamentally cannot achieve "privacy" on that device, because "thousands of random people seeing it" means it is not private, by definition. Sure, you can run Signal on that device, and it does actually run sound E2EE maths. But it is not an "end-to-end encrypted deployment", it is a public deployment that happens to be running E2EE cryptography.
What I say is that running E2EE in a browser has that kind of nuance (as far as the comparison goes, obviously). In that the browser is indeed running E2EE cryptography, but it is not an "end-to-end encrypted deployment", because the user has to trust the server.
> This is not a coherent segmentation for whether E2EE is possible and engaged.
I would argue that it is. Just like E2EE is not "possible and engaged" on the skyscraped in Manhattan, I argue that it cannot be in a browser. The browser is running sound cryptographic code, but what the user gets is not an end-to-end encrypted experience.